1. Field of the Invention
The present invention is generally directed to optical layer materials, especially for optical wavequide applications, and to the technique of producing such layers.
2. Description of Prior Art
The following prior art is considered:
(1) xe2x80x9cIntegrated Optics; Theory and Technologyxe2x80x9d, R. G. Hunsperger, Springer-Verlag 1984;
(2) Arnold et al., xe2x80x9cThin solid filmsxe2x80x9d, 165, (1988), p. 1 to 9, xe2x80x9cIon beam sputter deposition of low loss Al2O3 films for integrated opticsxe2x80x9d;
(3) Goell and Stanley, xe2x80x9cSputtered Glass Waveguides for Integrated Optical Circuitsxe2x80x9d, in Bell Syst. Tech. J. 48, 3445 (1969);
(4) M. D. Himel et al., xe2x80x9cIEEE Photonics Technology Lettersxe2x80x9d 3(10), (1991), p. 921 ff.;
(5) C. Henry et al., Appl. Optics, 26(13), 1987, 2621, xe2x80x9cLow Loss Si3N4xe2x80x94SiO2 Optical Waveguides on Sixe2x80x9d;
(6) J.Appl.Phys. 71(9), (1992), p. 4136, Grxc3xa4upner et al.;
(7) D-E-A-41 37 606;
(8) xe2x80x9cPlasma-Impulse CVD Deposited TiO2 Waveguiding Films: Properties and Potential Applications in Integrated Optical Sensor Systemsxe2x80x9d, Mat.Res.Soc., Spring Meeting San Francisco, 1992, Conference publication;
(9) xe2x80x9cMagnetron sputtering deposited AlN waveguides: Effect of the structure on optical propertiesxe2x80x9d, A. Cachard et al., Vacuum 41/numbers 4-6/p. 1151 to 1153/1990;
(10) Applied Optics, Vol. 14, No. 9, September 1975, New York, US, p. 2194-2198, Ingrey et al., xe2x80x9cVariable Refractive Index and Birefringent Waveguides by Sputtering Tantalum in O2/N2 Mixturesxe2x80x9d;
(11) Journal of Vacuum Science and Technology, Vol. 11, No. 1, January 1974, New York, US, p. 381-384, Westwood et al., xe2x80x9cEffect of Pressure on the Properties of Reactively Sputtered Ta2O5xe2x80x9d;
(12) Journal of Electronic Materials, Vol. 3, No. 1, 1974, US, p. 37-50, Cheng et al., xe2x80x9cLosses in Tantalum Pentoxide Waveguidesxe2x80x9d;
(13) Proceedings of the Spie: Hard Materials in Optics, Vol. 1275, Mar. 14, 1990, The Hague, NL, p. 75-79, Howson et al., xe2x80x9cThe Reactive Sputtering of Hard Optical Films of Tin Oxidexe2x80x9d;
(14) Journal of Vacuum Science and Technology: Part A, Vol. 2, No. 2, April 1984, New York US, p. 1457-1460, Demiront et al., xe2x80x9cEffects of Oxygen in Ion/Beam Sputter Deposition of Titanium Oxidesxe2x80x9d;
(15) Surface and Coatings Technology, Vol. 49, No. 1-3, Dec. 10, 1991, Lausanne, p. 239-243, Martin et al., xe2x80x9cDeposition of TiN, TiC, and TiO2 Films by Filtered Arc Evaporationxe2x80x9d.
It is known from (9) to deposit metal nitride layers or films by means of reactive DC-sputtering, namely layers of AlN. As optical waveguiding layers such layers are reported to have optical losses of about 11 dB/cm at minimum, at a wave-length of light of 633 nm and in the TE0-mode. Such layers are also reported to show optical losses down to 5 dB/cm.
It is known from (4) to manufacture TiO2 which, applied as material of a waveguiding layer, exhibits optical losses lower than 10 dB/cm. Thereby it is not specified for which wave-mode and for which wave-length of light such losses are valid. From this reference it is further known to apply Ta2O5 for waveguiding layers which exhibit optical losses of less than 5 dB/cm, which losses are again neither specified with respect to wave-lengths of light nor with respect to propagation mode. The layers are here produced by an ion plating technique.
In agreement with the contents of (4), even in the year 1991, the reference (7) teaches that TiO2 would be most suited as a material to produce thin film optical waveguides due to its physical and chemical properties. In spite of titanium oxide exhibiting a very high index of refraction, a good chemical resistance and being very hard, it is reported that no method had been known in the literature for producing a low loss titanium oxide thin film waveguide, because of, the fact that titanium oxide exhibits a high tendency toward crystallization during manufacturing.
Therefore the reference (7) proposes to deposit TiO2 as a material suited as optical waveguiding material by means of a pulsed micro-wave plasma CVD-method. When applied as a material for waveguiding purpose, the TiO2 produced by the method proposed in (7) exhibits for TE01-waves of a wave-length, optical losses of about 2.5 dB/cm.
With respect to wave-lengths it is principally valid that the optical losses become the larger, the shorter the wave-length selected.
From the reference (2) it is further known to produce Al2O3-layers by ion beam sputtering, exhibiting low optical losses, lower than 1 dB/cm, with no propagation mode and no wavelength of light specified. Due to the proposed ion beam technology, the proposed manufacturing method is not suited for large areal coating and exhibits a relatively low coating rate. This in combination results in an accordingly uneconomic layer production.
The reference (3) proposes to use as a material for optical waveguiding layers, Rf-sputtered glass. The reference (5) further proposes to produce a material which is suited for waveguiding applications by means of low pressure plasma CVD followed by a heat treatment annealing step.
The reference, (8) further proposes to produce TiO2 by means of plasma impulse CVD, which material, applied for monomode waveguiding in the TE0-mode, exhibits optical losses of 2.4 dB/cm or of 5.1 dB/cm in the TM0-mode, each referred to the wave-length of light of 633 nm.
In spite of the knowledge out of reference (9), the reference (6) still describes that reactive sputtering of metal nitride layers, namely of AlN, from metallic target results in layers which, applied as optical waveguiding layers, exhibit very high optical losses of 300 dB/cm at propagation conditions which are not specified. Such a material is, in fact, not any more an optical layer material due to its extremely high optical losses and may especially not be said to be suited for optical waveguiding.
Other metal oxides such as TiO2 in reference (7), would be suited as optical layer material, whereby known methods for producing layers of such materials, as e.g. ion beam sputtering according to reference (2), micro-wave plasma CVD according to reference (7), plasma impulse CVD according to reference (8), low pressure plasma CVD according to reference (5) or ion plating methods according to (4), are disadvantageous especially with respect to large areal coating and deposition rate, so that the wide-spread production of such layer materials is very difficult to reduce to practice in a commercially feasible manner.
The recognition published in reference (7), according to which TiO2 has the tendency of crystallization during its production, is made, with respect to tantalum pentoxide, in reference (10), i.e. in the year 1975. According to (10), already in that year, reactive DC-diode sputtered optical waveguiding layers were proposed, sputtered in N2- and O2-gas mixture atmosphere. Thus in fact, some sort of tantalum oxinitride layers were proposed.
For deposition rates of approx. 0.4 xc3x85/sec and at temperatures of about 200xc2x0 C., there are reported optical losses in the TE0- and the TM0-modes. lower than or equal to 1 dB/cm at a wavelength of light of approx. 633 nm. Such results are attributed to the nitride addition to the sputtering atmosphere.
From the reference (11) of 1974, which is referred to in reference (10), it is known to produce Ta2O5-layers for thin film capacitors and for optical waveguides by means of reactive DC-diode sputtering in an O2/Argon atmosphere. Different sputtering parameters are varied and losses of approx. 1 dB/cm are reported from the best layers thus produced. Thereby, the following dependencies are reported:
With rising sputtering pressure:
increase of the optical losses;
increase of coating rate;
reduction of coating temperature.
The temperatures reported in reference (11) are in a range between 160xc2x0 C. and 350xc2x0 C. at lower pressure of approx. 1.6xc2x710xe2x88x922 mbar operating pressure and are about 180xc2x0 C. at higher operating pressure of about 8xc2x710xe2x88x922 mbar.
In reference (12), which is referred to in (10) as well as in (11) and which has in parts the same authors, comparisons are made between Ta2O5-layers produced by different methods, so e.g. deposited by reactive DC-sputtering or by metallic sputtering with thermal post oxidation at temperatures of between 550xc2x0 C. and 650xc2x0 C. For Ta2O5-layers deposited by reactive DC-sputtering, optical losses between 1 and 6 dB/cm are reported for the TE0-mode, such layers being produced at a deposition rate of approx. 0.12 xc3x85/sec and at deposition temperatures of 200xc2x0 C.
In combining the references (12) up to (10), there may be seen the tendency of leaving the approach of manufacturing low loss tantalum pentoxide layers and trying to manufacture such layers rather from Tantaloxinitride, deposited by reactive diode-DC-sputtering at relatively low deposition rates and at relatively high deposition temperatures.
The reference (13) further describes production of SnO2-layers by DC-sputtering. The measuring results published in this reference lead to the conclusion that the optical losses will be in the range of 3xc2x7104 dm/cm.
From (14) it is further known to produce TiO2-layers by means of ion beam sputtering. A rough estimate of the optical losses from the published measuring results leads to such losses in the range of 400 dB/cm.
Finally, the reference (15) describes to produce TiN-, TiC- and TiO2-layers by means of arc evaporation. From the extinction coefficient which is reported for TiO2-layer material of 0.07 at a wave-length of 633 nm, very high optical losses may be expected.
It is a first object of the present invention to provide a material for optical layers of a metal oxide, production of which may be significantly less expensive, which exhibits low optical losses, which further may be produced at lower temperatures compared with temperatures at which such known layer materials are produced.
This object is realized by an optical film material of a metal oxide being deposited by reactive magnetic field enhanced DC-sputtering from a metallic target and having optical losses of 15 dB/cm at the most for a wave-length of light of 633 nm.
By reactive magnetic field enhanced DC-sputtering, the expected low optical losses are maintained and, additionally, high deposition rates at low deposition temperatures are reached. Under the expression xe2x80x9cmagnetic field enhanced sputteringsxe2x80x9d all DC-sputtering techniques shall be understood, at which lines of magnetic flux are generated, which loop in a tunnel-like pattern across the target surface and/or which loop from the target surface to neighbouring parts of a processing chamber. An especially preferred example of such magnetic field enhanced DC-sputter techniques is magnetron DC-sputtering.
It is a further object of the present invention to provide such optical layer material of further reduced optical losses, which is realized by reducing the optical losses mentioned to 4 dB/cm at the most.
It is a further object of the present invention to provide said optical film material with high chemical resistance and with high hardness, which object is achieved by the optical film material mentioned above, being TiO2, with even further reduced losses of 1.5 dB/cm at the most.
The last mentioned object of the present invention is further realized, too, by providing the optical film material mentioned, as tantalum pentoxide, with reduced optical losses of 3 dB/cm at the most.
It is a further object to provide the optical film material of metal oxide mentioned above with even further reduced losses, which is realized by such a material having optical losses at the wave-length mentioned of 1.5 dB/cm at the most.
Following the further object to even further reduce the optical losses as specified, it is proposed to provide the optical film material of metal oxide with optical losses of 0.7 dB/cm at the most or even of 0.3 dB/cm at the most.
It is a further object of the present invention to propose the mentioned optical film material so that it may be produced in a controlled process in the sense that arcing, as may occur in reactive DC-sputter deposition of metal oxide, may strictly be maintained under control. This object is realized by the mentioned optical film material being deposited by time-intermittent reactive magnetic field enhanced DC-sputtering.
It is a further object of the present invention to provide an optical film material as mentioned above which is produced economically, which is resolved by the material of the inventive optical film being deposited with a deposition rate of 0.5 xc3x85/sec at the minimum and even with a deposition rate of 0.9 xc3x85/sec at the minimum.
It is a further object of the present invention to propose an optical film material which is deposited so that e.g, a. substrate, whereon it is deposited, is not affected by the deposition process. This object is realized by such material being subjected to a deposition temperature of 150xc2x0 C. at the most and even to a deposition temperature of 100xc2x0 C. at the most and, in a further improved embodiment, to a deposition temperature of 70xc2x0 C. at the most.
It is a further object of the present invention to provide TiO2-material exhibiting very low optical losses, which object is realized by TiO2 having optical losses of 1.5 dB/cm at the most for a wave-length of light of 633 nm. Thereby, an even improved TiO2 exhibits optical losses at the said wave-length of light of 0.7 dB/cm at the most and even of 0.3 dB/cm at the most.
It is a further object of the present invention to provide a Ta2O5-material with optimized optical losses, which is realized by Ta2O5 wherein the optical losses for light of the wave-length of 633 nm are 3 dB/cm at the most.
Thereby, an even further improved Ta2O5 is proposed, wherein the optical losses specified above are 1.5 dB/cm at the most or even in a further improved material are 0.7 dB/cm or even 0.3 dB/cm at the most.
It is a further object of the present invention to provide an optical layer which is produced in an economically satisfying manner and is thus producible on a large scale. This object is achieved by an optical layer made of an optical layer material of a metal oxide, being deposited by reactive magnetic field enhanced DC-sputtering of a metallic target and having optical losses of 15 dB/cm at the most for light of the wave-length of 633 nm.
The object of even further reducing the optical losses of such optical layers is achieved by providing TiO2 as layer material with optical losses being 1.5 dB/cm at the most or even being 0.7 dB/cm at the most.
Following the object of further improving the optical layer mentioned above and providing for a further layer material to that target, said optical layer of tantalum pentoxide is proposed with optical losses of 3 dB/cm at the most or even with such losses of 0.7 dB/cm at the most.
It is a further object of the present invention to construe by means of the optical layer mentioned above an optical multi-layer system. This is realized by the optical layer mentioned above within an optical multi-layer with at least one optical layer of lower refractive index material and at least one optical layer of higher refractive index material, whereby the optical layer which is deposited by reactive magnetic field enhanced DC-sputtering is the higher refractive index material layer.
It is a further object of the present invention to provide the optical layer mentioned above as an optical waveguiding layer which is achieved by that optical layer, being an optical waveguiding layer with the low optical losses specified above, being valid in a TM-monomode, especially in the TM0-mode.
In a preferred embodiment for a large number of applications, the inventive optical layer is a substantially flat optical waveguiding layer.
It is a further object of the present invention to propose a method for producing a layer of a metaloxide resulting in economic production of such layers and in layers with low optical losses. This is realized by a method for producing a layer of metaloxide with optical losses of 15 dB/cm at the most for light of a wave-length of 633 nm, which method comprises magnetic field enhanced reactive DC-sputter deposition of the layer.
In a first preferred way of execution, the inventive method is performed by the mentioned sputtering realized in the oxide mode. To fulfil the object of further optimizing the inventive method of production, performing the reactive magnetic field enhanced DC-sputtering, is made in the instable transition mode (between metallic mode and oxide mode) and the transition mode of processing is stabilized to lead to a stable processing working point.
Thereby, it is further proposed to perform the magnetic field enhanced reactive DC-sputtering in a time-intermittent manner, preferably in a time-intermittent manner at a frequency of 30 kHz at the most and in a further preferred manner at a frequency of 20 kHz at the most.
All magnetic field enhanced reactive DC-sputtering mentioned up to now is preferably performed as magnetron sputtering.
Further objects and their solutions will become apparent to the man skilled in the art by the wording of the claims and of the following description of the invention.
The invention is, additionally to the following examples and the above general description, insofar as may be necessary for the man skilled in this specific art, described by means of one drawing.
The invention will thus be better understood and objects other than those set forth above will become apparent when consideration is given to this drawing, the description thereof and the described examples.